In the art, various systems to adjust the backrest are known, and particularly to adjust the tilt thereof.
In some systems, it is possible to adjust the backrest in different fixed positions. In these cases, a user adjusts the backrest tilt according to its own needs, and then imposes that this position is kept whereby the backrest holds the position regardless of the force the user applies on it, and particularly the force the back of a user using the chair applies.
Systems providing greater use portability are known in the art.
For example, chairs are known provided with elastic means, typically torsion springs, to counterbalance the weight of the user against the backrest. In such systems, when the user leans against the backrest, the latter tilts so as to deform the spring from its resting condition, compressing or tensioning it. As known, the higher is the compression or tension of the spring with respect to its undeformed state, the higher is its resistant force, whereby the backrest tilts until it reaches an equilibrium position where the user weight is counterbalanced by the spring resistance. Alternatively, the use of gas springs is known, operating according to the same principle, whereby a higher compression of the gas spring involves a higher resistant force thereof.
These systems are effective only if the user weight is within a given range. Out of this range, for example if the user is very light or very heavy, the above mentioned equilibrium position is quite uncomfortable, since in this position the backrest is too much or too little tilted.
Therefore, further systems are known allowing to adjust the resistance provided by the spring.
In EP1874161, the backrest of a chair is integral to a rigid element whose rotation is countered by a spring action. A third class lever is interposed between the spring and the rigid element, whereby the lever end is hinged to a fulcrum and the other end is the point of application of the spring resistant force. The force operating on the backrest is applied in a point of application coincident with a slider which can move along the lever. Therefore, by varying the lever arm with respect to the pin, the resistance provided by the lever operated by the spring varies. Moreover, the slider can also move along a surface of the rigid element, which works in its turn as a lever, so as to vary the lever arm along the rigid element.
Such a system actually allows to adjust the resistance provided by the spring, allowing the users having a weight different from the standard to get a comfortable position on the chair. However, the construction of these systems may be quite complicated and, further, they do not allow a precise adjustment of the spring resistance. Due to construction, the mentioned lever has a limited length whereby a very little displacement of the pin causes the spring resistant force to widely vary, thereby a precise adjustment becomes difficult for the end user.
Similarly, WO 2010/103554 describes a system for varying the force that opposes the movement of a backrest by varying the working angle of a spring. Particularly, at one end the spring is pivoted on a fixed pin, and at the other end on a mobile pin slidable within a slot. By adjusting the position of the mobile pin along the slot, it is possible to vary the lever arm used by the backrest to work the spring.
Such a solution is not able to solve the problems of EP1874161.